1. Field of the Invention
The present invention relates to an ink jet printing apparatus and method to eject ink for printing according to a print signal and more particularly to an ink jet printing apparatus and method having a double-sided printing function.
2. Description of the Related Art
As information processing equipment, such as copying machines, word processors and computers, and communication equipment proliferate, ink jet printing apparatus are becoming increasingly widespread as one of image forming output devices for these equipment. Ink jet printing apparatus employ an ink jet system that ejects ink from orifices of a print head to form a digital image. The print head of such printing apparatus has a large number of ink ejection nozzles and ink paths integrated therein at high density for high resolution printing. In recent years, there are growing calls for printing color images. Many ink jet printing apparatus meet this demand for color image printing by mounting a plurality of print heads ejecting different colors of ink.
The ink jet printing system performs a dot printing by ejecting ink as a recording liquid in the form of flying droplets to land on a print medium such as paper. That is, the ink jet printing system is a non-contact printing system in which the print medium and the print head are kept out of contact with each other during printing, so its operating noise is low. Further, the ink jet printing system has advantages of being able to realize a high resolution and a fast printing speed by increasing the density of ink ejection nozzles and to produce a high-quality image at low cost without requiring special processing, such as fixing, on a print medium including plain paper. Because of these advantages, the ink jet printing apparatus are in wide use today. An on-demand type ink jet printing apparatus can easily be upgraded to print color images and easily made compact in size and simple in construction. It is therefore expected to have an increased demand in the future.
The use of the printing apparatus on the part of user has diversified. To meet diversified uses, a printing apparatus has become available which provides a variety of printing modes. One such example is an ink jet printing apparatus with a double-sided printing mechanism that allows for continuous printing on both sides of a print medium.
In the printing apparatus with such a double-sided printing mechanism, a print medium is normally applied with ink on one of its print surfaces (hereinafter referred to as a front surface or first print surface) to form an image and then is turned over before being printed on the other surface (referred to as a back surface or second print surface).
In this double-sided printing apparatus, if, after the front surface has been printed, the print medium is turned over without the ink on the front surface being thoroughly dried, there is a possibility of the print medium starting to be fed and printed before a cockled or waved state of the print medium is eliminated. In that case, the print head may come into sliding contact with the print medium, failing to produce an image of an intended quality. Further, as the print medium is reversed or the back surface is printed, a waved portion of the print medium may get caught in a transport mechanism, causing a jam.
It is therefore desired that, after the printing of the front surface, the ink applied to the print medium be thoroughly dried so that the turning over of the print medium will not result in a jam or a head-medium rubbing. That is, a standby time, or a time it takes from when the front surface printing is finished until the print medium begins to be turned over, should preferably be set considering the time required to dry ink.
As a conventional means for setting the standby time, Japanese Patent Application Publication No. 2879872 discloses a printing apparatus which determines the standby time according to the kind of print medium. Japanese Patent Application Laid-open No. 2003-048311 discloses a printing apparatus which determines the standby time according to a print duty.
In Japanese Patent Application Publication No. 2879872, since the length of standby time is determined by the kind of print medium, equal standby times are set for those print mediums of the same kind even if they are printed at different print duties. Hence, when the print duty is high, the set drying time may be too short for some of the print sheets; and when the print duty is low, too long a drying time may be set, wasting time.
To get around this problem, the printing apparatus in Japanese Patent Application Laid-open No. 2003-048311 determines the standby time according to the print duty. In Japanese Patent Application Laid-open No. 2003-048311, however, since the standby time is determined based on the print duty of the entire surface, equal standby times are set as long as the print duties are equal whatever the distributions of ink quantity applied to the surfaces may be. For example, whether an image formed on the surface is deviated to one side or to the center, the standby times are set equal as long as the overall ink application quantities to one page are the same.
A jam and a head-medium rubbing, based on which the standby time is determined, are likely to occur near the end portions of print medium. Hence, ink applied to near the end portions of print medium needs to be dried sufficiently. On the other hand, it is not necessary to provide much drying time for the ink applied to the central portion.
As described above, since the standby time from the end of the front surface printing to the start of the back surface printing has conventionally been determined simply by the amount of ink applied to the entire surface without taking into account the distribution of ink application amount, the printing time has not been reduced sufficiently. That is, because a long standby time is set even when a large amount of ink is distributed to those parts of a print medium where the ink is not likely to cause such problems as jam and head-medium rubbing, the printing time is unduly long. As demands are growing now for faster printing speed and higher print quality, it is desirable to eliminate an excess, unused portion of the above-described standby time to reduce the printing time while at the same time minimizing the occurrence of jam and head-medium rubbing.